This invention concerns a process for monitoring nitrogen processes. The invention concerns in particular a method for monitoring nitrogen processes in semiconductor manufacture.
In the manufacture of integrated semiconductor components there is frequently a need for nitrogen processes for the fabrication of nitride layers and/or nitrogen processes for the insertion of nitrogen into already existing layers of material. Typical examples of nitrogen processes of that kind are the fabrication of a silicon nitride layer as a memory dielectric or as part of the memory dielectric in storage capacitors, the fabrication of a silicon nitride layer as a mask layer for the LOCO method, or the fabrication of silicon nitride layers as so-called xe2x80x9cpad nitride,xe2x80x9d which are used as stop layers for etching or polishing. Furthermore, nitride layers and oxynitride layers are used as tunnel layers at the boundary layer between a buried contact and the conductive filling of a trench capacitor. A tunnel layer of that kind is described in the commonly assigned European patent application EP 0 977 266 A1, which is herewith incorporated by reference (see also, copending applications Ser. Nos. 09/232,081 and 09/363,277, which are also incorporated by reference). The fabrication of an exactly defined nitride or oxynitride layer is of especially great importance in the last-named application. The specifications set for the nitrogen processes used must therefore be correspondingly high.
In prior art, the nitrogen processes were monitored as a rule using secondary ion mass spectrometry (SIMS) or Auger electron spectroscopy (AES). In secondary ion mass spectroscopy the surface to be tested, in this casexe2x80x94for examplexe2x80x94a nitride or oxynitride layer, is bombarded under high vacuum with a primary ion beam (O, Cl, Ar, or other gases) which causes ionized particles to be ejected from the surface. These secondary ions are then analyzed in a mass spectrometer. Secondary ion mass spectrometry has the advantage that all elements (including isotopes) can be detected in concentrations down to the ppb range. In this way conclusions can be drawn about the nitrogen content of the layer under investigation, and therefore about the quality of the nitrogen process used.
In Auger electron spectroscopy the sample surface is bombarded with a primary electron beam, through which the atoms at the sample surface are ionized in a low-lying energy level (K or L shell). If the hole in the low-lying energy level is reoccupied by an electron from a higher-lying energy level, the energy released can be transferred without radiation to a further electron in a higher energy level, which then leaves the atom as a so-called xe2x80x9cAuger electron.xe2x80x9d Measurement of the kinetic energy of the Auger electrons enables the kind of atom in the sample surface to be determined. Since Auger electrons emitted from a sample surface are only generated at a very shallow depth, Auger electron spectroscopy is a very powerful method for analyzing the thinnest of surface layers.
Unfortunately, both secondary ion spectrometry and Auger electron spectroscopy are very complicated and therefore very costly methods, which cannot normally be used in a production environment.
The object of the present invention is to provide a method for monitoring nitrogen processes which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which reduces or completely avoids the above disadvantages. In particular, it is the object of the invention to provide for a method for monitoring nitrogen processes which is capable of determining the quality of the nitrogen process in a simple manner and at reasonable cost.
With the above and other objects in view there is provided, in accordance with the invention, a method of monitoring nitrogen processes, which comprises the following steps:
providing a silicon surface;
subjecting the silicon surface to a nitrogen process;
producing an oxide layer on the silicon surface by thermal oxidation for a specified duration; and
determining a thickness of the oxide layer as a measure for a quality of the nitrogen process.
The novel method for monitoring nitrogen processes makes use of the fact that the incorporation of nitrogen near the surface in the silicon, or in a thin silicon nitride layer on the surface of the silicon, inhibits oxygen diffusion during the subsequent thermal oxidation. Correspondingly, the oxidation rate of the thermal oxidation is reduced and the growth of the oxide layer on the silicon surface is inhibited. The thickness of the oxide layer can therefore be used as a measure for the nitrogen content, i.e. for the quality of the nitrogen process.
The prior art methods used for monitoring nitrogen processes (SIMS, AES) are based on direct observation of the nitrogen atoms. However, such direct proofs of a particular type of atom are very complicated. The method according to the invention does without a direct proof of the nitrogen atoms, whereby the monitoring of nitrogen processes can be performed considerably faster and at less cost. The method according to the invention also has the advantage that the test wafer used as a rule for the monitoring is not destroyed as a result of this monitoring and can therefore be used as a so-called xe2x80x9cdummyxe2x80x9d in further processes.
According to a preferred implementation, the oxide layer is fabricated through a thermal oxidation at a temperature between 800xc2x0 C. and 1100xc2x0 C., preferably at 900 C. In addition, it is preferred if the oxide layer is fabricated through a thermal oxidation over a duration of 10 to 30 minutes, preferably 20 minutes. In doing this, it is especially preferred if the oxide layer is fabricated as wet oxide.
The thickness of the oxide layer can be determined using several different methods. For example, the thickness of the oxide layer can be measured using laser interferometry. According to a further preferred implementation the thickness of the oxide layer is determined by ellipsometry. Moreover, it is preferred if the silicon surface is cleaned using a wet chemical method before the nitrogen process and, especially, if the so-called xe2x80x9cnatural oxidexe2x80x9d is removed from the silicon surface.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for Monitoring Nitrogen Processes, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.